Alterations of the tumor suppressor p53 are the most commonly identified mutations in human cancers. In responses to various stresses, p53 protein level and its activity are greatly induced. However, the mechanism how p53 responses to various stresses are activated largely remains unclear. Recent studies have suggested that various phosphorylation events of p53 might regulate p53 stability and activity. However, the physiological roles of these phosphorylation events of p53 in regulating p53 responses to DNA damage and other stresses remain to be determined. To address this issue, I propose to employ homologous recombination and LoxP-Cre-mediated deletion to introduce missense mutations (Ser/Thr to Ala mutation) at several potentially important p53 phosphorylation sites, including Ser18 and Thr73/83, into the endogenous p53 in mice. Preliminary analysis of the p53(Ser18Ala) and p53(Thr73/83Ala) primary cells suggested that both phosphorylation events play important but distinct roles in regulating p53 stability and activity after DNA damage. The mechanism for the impaired p53 responses to DNA damage in these p53 knock-in mice and the effects of these mutations on the p53-dependent tumor suppression will be determined. In addition, employing the same approach, we will determine the potential functional redundancy between phosphorylation of p53 at Ser18 and ser23 in regulating p53 responses to DNA damage. Phosphorylation of human p53 at Ser46 has been suggested an important role in regulating p53 apoptotic function. However, Ser46 of human p53 is not conserved in mouse p53. Therefore, a human p53 knock-in mouse model, in which exons 4-9 of mouse p53 gene was replaced with exons 4-9 of human p53 gene, will be used to address the physiological roles of this phosphorylation event. Two observations indicated the feasibility of this strategy. First, the humanized p53 is functionally equivalent to the endogenous mouse p53. Secondly, the DNA damage-induced signaling pathways leading to the phosphorylation of human p53 at Ser46 is conserved in mouse cells. Identification of the phosphorylation events that regulate p53 stability and activity will indicate the signaling pathways involved and thus reveal the mechanism how p53 responses are activated during various stresses or cellular senescence.